1. Field of the Invention
The present invention relates to a controller for a synchronous motor used for a machine tool, and so on, and particularly to a controller for a reluctance type synchronous motor.
2. Description of the Related Art
A permanent magnet type synchronous motor using a permanent magnet for a rotor has conventionally been used for positioning a machine tool or driving a feeding spindle of a machine tool. When applying an electric current to such a motor, only the amplitude and phase of an armature current (torque current) must be controlled. In this motor, however, as the magnetic force of the magnet cannot be controlled, the magnetic field cannot be desirably controlled using, for example, a field weakening control (a constant output). Consequently, the inductive voltage between terminals exceeds the power source voltage, becoming saturated when the rotation speed reaches or exceeds a designed rotation speed (hereinafter referred to as a base rotation speed), causing a problem of unstable control.
Therefore, instead of a permanent magnet type motor, a reluctance type electric motor capable of using independently controlled field and armature currents is used. With such a motor, a field current is weakened in accordance with a rotor speed (which is equivalent to reduction of the magnetic force of a magnet) when the rotor speed reaches or exceeds the base rotation speed, so that control can be made stable even when the rotor speed is equal to or higher than the base rotation speed.
FIG. 15 is a block diagram showing an example of a conventional controller for a reluctance type synchronous motor.
An adder 1 obtains a speed difference DIF based on a speed command SVC and a rotor speed SPD, and outputs the obtained speed difference DIF to a PI controller 2, the speed command SVC being instructed by a higher order controller (not shown) and the rotor speed SPD being obtained through conversion by a differentiator 11 using a rotor position SP read by a detector 10 mounted on a motor 6. The PI controller 2 then obtains a torque command STC by multiplying the speed difference DIF and a speed loop gain to output to a torque command-current converter 7. The torque command-current converter 7 then performs level conversion in a level converter 71, so that the torque command STC is converted into an armature current amplitude command SIQ. After multiplication by an output from a phase distributor 73 in a multiplier 72, a resultant armature current command SIAC is output to an adder 3.
Meanwhile, in the field current calculator 9, a field current coefficient calculator 91 outputs, referring to the rotor speed SPD, a field current coefficient SKD in accordance with a function pattern 21 shown in FIG. 2(a) (a curved line function taking a constant value with a rotor speed equal to or lower than a base rotation speed Nbase, and depending on a rotor speed SPD equal to or higher than a base rotation speed Nbase). The output coefficient SKD is multiplied by a default field current IDC in a multiplier 93 so that the resultant field current amplitude SID is output to a multiplier 94. In the multiplier 94, the field current amplitude SID is multiplied by an output from a phase distributor 95, so that the resultant field current command SIFC is output to the adder 3. The phase of the field current command SIFC is displaced by .pi./2 from that of the armature current command SIAC as a result of processing by a phase converter 97 and an adder 96. The adder 3 performs vector addition using the armature current command SIAC and the field current command SIFC, so that a resultant combined current command SIC is output to a phase distributor 4. Thereafter, current commands SIUC and SIVC with respective phases are amplified in an amplifier 5, and the result is output to drive the motor 6.
In the above conventional controller for a reluctance type synchronous motor, field current control depends solely on the rotor speed SPD, and field weakening control is applied only when a rotor speed is equal to or more than the base rotation speed Nbase. Moreover, because of an arrangement in which a large field current flows when a large toque is necessary, a field current always flows even when no torque is required, e.g., when the motor is stopped or operating at a constant speed. This leads to problems of a large power consumption and heat generation of the motor. On the other hand, when a field current is small, a torque constant becomes small, which only enables generation of a smaller torque for the same armature current. This leads to problems of a prolonged motor acceleration time and, when applied to amachine tool, a prolonged machining cycle. Moreover, if an armature current is large in view of a field current, armature reaction may be caused, deteriorating torque linearity with respect to an armature current and, as a consequence, lowering a torque constant.
The influence of the drop in torque constant may appear as machining fringes on a work piece being cut, or deteriorating machining accuracy.